When all of a sudden you realize something terrible has happened to you, it feels like being slapped in the face with a wet fish. In instances where you gain this insight right after waking up, you have obviously 'slept with a wet fish' only to realize what a fool you've been after you wake up.
When I woke up and realized those conmen had tricked me out of a lot of dough last night, I really knew I had slept with a wet fish all nightlong!
Referencing well-known medical drama 'Grey's Anatomy,' GSWI is the sudden onset of remembering events, usually regrettable, that occured previously while heavily intoxicated, just as the late George O'Malley remembered his adulterous sex with Isobel Stevens.